1. Field of Invention
The present invention relates to an improvement to a cathode for use in electrorefining or electrowinning of copper. In particular, the present invention relates to an apparatus and method for plating copper onto a stainless steel sheet that may be attached to a solid copper head bar.
2. Description of the Related Art
Electrorefining is one of the final steps in copper processing. Generally, copper anodes and starting sheets are disposed in an electrolytic solution that comprises of copper sulfate and sulfuric acid. The sheet acts as a cathode and is typically made of stainless steel to deter corrosion during the process. An electric current is passed through the solution causing copper from the positively charged anode to be deposited on the negatively charged sheet. Electrowinning is performed by a similar process, but the copper is removed from solution, rather than from an anode, and deposited on the sheet. When an appropriate amount of copper has been deposited on the sheet, the sheet is removed from the solution and stripped of the copper.
For example, one such method of electrorefining is performed by using a cathode developed by Dremco Specialized Products for Electrowinning and Electrorefining and manufactured by T. A. Caid in Arizona. This method is disclosed in U.S. Pat. No. 5,492,609 (incorporated herein by reference). The stainless steel sheet in this cathode is attached to the copper head bar by means of a filet welding along a horizontal groove on the underside of the head bar after the upper edge of the stainless steel sheet is inserted into the groove. The weld size is around three to four millimeters wide using copper filler metal.
However, this welding formulation has several problems and disadvantages. Dissimilar metal fusion welding involves the melting and mixing of metals, usually with the addition of a filler metal. Because the metallurgical and physical properties of copper and stainless steel differ significantly (e.g., copper and stainless steel have different melting points, thermal and electrical conductivities, and coefficients of thermal expansion), certain problems arise as a result of the welding.
For example, the substantial difference in the melting temperatures of copper and stainless steel causes segregation of the base metals upon weld solidification. The higher melting temperature metal will solidify first, leaving the lower-melting constituents segregated. Also, the variation in the coefficients of thermal expansion (CTE) of the metals can cause the development of stresses due to shrinkage. This can cause hot cracking problems. Moreover, penetration of liquid copper into the stainless steel grain boundaries can occur and this is known to cause solidification cracking.
There is another problem that arises with the welding of copper and stainless steel. Copper and stainless steel are resistant to copper electrolyte. However, the weld metal also uses a copper filler metal, which is a mixture of stainless steel and copper. This mixture has a lower corrosion resistance than the parent metals. Therefore, because the weld metal is neither copper nor stainless steelxe2x80x94but is instead a mixture of bothxe2x80x94it has a lower corrosion resistance than its parent metals. In addition, it is possible that galvanic corrosion will occur in the weld metal due to the segregation of copper and stainless steel in the weld metal.
Finally, yet another drawback of existing cathodes is the occurrence of crevice corrosion of the stainless steel inside the groove. Crevice corrosion of stainless steel occurs when electrolytes enter into the groove in the gap between the stainless steel sheet and copper hanger bar as a result of weld corrosion.
These problems lead to cracking at the ends of the weld joint between the copper hanger bar and stainless steel sheet. Finite element analyses of the cathodes currently in use reveal that both ends of the cathodes are exposed to high stress concentrations. These high stresses are the result of operating parameters that include exposing the weld joint to various impacts. For example, the cathodes are subjected to both static and dynamic loads during transportation, as well as during the processes of stripping the deposited copper off the stainless steel sheet.
Furthermore, continued dissolution of weld metal due to corrosion and its subsequent separation from the stainless steel reduces the amount of contact surfaces between the stainless steel sheet and copper head bar. Crevice corrosion of the stainless steel sheet inside the groove also reduces the amount of contact between the stainless steel sheet and copper head bar. These reductions in contact area negatively affect the current density, which in turn negatively affects the rate of production.
It is an object of the present invention to address some of the above-listed problems and thereby prolong the life of the cathode.
Briefly, according to the present invention, the cathode has a groove that is preferably deeper than the known cathode on the underside of the head bar. A stainless steel flat sheet with several punched holes is preferably inserted into the groove, and then the copper head bar can be pressed against the stainless steel sheet at the points where the stainless steel sheet is perforated. As a result of localized pressing, copper extrudes into the holes in the stainless steel sheet. This ensures contact between the copper head bar and stainless steel sheet and lessens the role of the weld joint between the bar and the sheet.
In one aspect of the present invention a cathode is provided for electrolytic copper refining. The cathode comprises a copper head bar having a horizontal groove on an undersurface and a stainless steel sheet that has a plurality of holes at an upper end thereof, the upper end of said sheet being disposed in the horizontal groove of said copper head bar and copper from said copper head bar being disposed inside the plurality of holes at the upper end of said sheet to form a mechanical bond between said copper head bar and said sheet.
In another aspect of the present invention a method for making a cathode is provided. This method comprises the steps of (1) inserting a stainless steel sheet, said sheet comprising a plurality of holes at an upper end thereof, into a copper head bar by placing the upper end of said sheet into a horizontal groove located on an undersurface of said copper head bar and (2) pressing said copper head bar against said sheet in such a manner that copper from said copper head bar flows into the plurality of holes at the upper end of said sheet causing a mechanical bond between said copper head bar and said sheet.
In yet another aspect of the present invention there is provided a method for making a cathode comprising the steps of (1) forming a plurality of holes at an upper end of a stainless steel sheet, (2) forming a horizontal groove in an undersurface of a copper head bar, (3) inserting the upper end of said sheet into the horizontal groove of said copper head bar, and (4) localized pressing of said copper head bar at locations that correspond to the locations of the plurality of holes in said sheet.
Other objects, features, and advantages of the present invention will become apparent from the following detailed description taken in conjunction with the accompanying drawings.